Automatic gain control circuit



une J l8, 1937 w. R. KOCH -Y 2,083,025

` AUTOMATIC GAIN CONTROL CIRCUIT Original Filed July 6, 1934 2 Sheets-Sheet 1 T0 Aam/0 /VEYWURK A. u c. kfcr/F/E/a ATTORNEY.

June 8, 1937. w. R. KOCH A AUTOMATIC GAIN CONTROL CIRCUiT Original Filed July 6, 1934 2 SheetS-Sheet 2 ATTORNEY.

Patented June 8, 1937 UNITED STATES AUTOMATIC GAIN CONTROL CIRCUIT Winfield R. Koch, Merchantville, N. J., assignor to Radio Corporation of America, a corporation of Delaware Original application July 6, 1934, Serial No.

733,987. Divided and this application November 30, 1935, Serial No. 52,327`

2 Claims.

One of the main objects of the present inveni tion is to provide automatic gain control circuits for signal transmission tubes, whether of high frequency or audible frequency, which circuits embody control tubes having output electrodes adapted to be energized by alternating current energy, the alternating current energy being of intermediate frequency, or audible frequency.

Another important Object of the present invention is to provide automatic volume control circuits for radio receivers wherein the automatic volume control tubes may have their plates energized by alternating current energy, in place of the usual direct current sources, and the alternating current energy being derived from an existent source in the' receiver.

Another object of the invention is to provide a background noise suppressor arrangement for a superheterodyne receiver wherein the suppressor tube is adapted to operate with alternating cur.

rent upon its plate circuit, and the input to the suppressor tube is sharply tuned to the operating intermediate frequency.

Still other objects of the invention are to improve generally the efficiency of automatic gain control circuits for radio receivers, and more especially to provide in radio receivers automatic volume control circuits, as well as interchannel noise suppressor circuits, which are not only reliable in operation, but readily assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit or` ganizations whereby my invention may be carried into effect.

In the drawings:

Fig. 1 shows a form oi the invention applied to an interchannel noise suppressor circuit as embodied in a superheterodyne receiver,

Fig. 2 shows a modification of the system shown in Fig.`1. l

Referring now to the accompanying drawings, wherein like reference characters in the differ- CII ent figures correspond to similar circuit elements, there is shown in Fig. 1 an automatic volume control system for a superheterodyne receiver particularly. It is to be clearly understood that the nature of the receiver with which the volume control network is associated is of comparatively little importance as far as the present application isconcerned. The superheterodyne type of receiver has been employed as an illustration because it is at the present time substantially universally employed in broadcast reception in the United States and in many foreign countries. For this reason the various networks of the receiver are conventionally shown in Fig. 1, and it will be observed that an R. F. amplifier feeds signals to a first detector upon which is impressed energy of local oscillation frequency from a local oscillator. The intermediate frequency output of the detector network is impressed upon an intermediate frequency amplifier.

The signal input energy to the first detector may be connected to a signal collector, or to one or more stages of tuned radio frequency amplification. The amplified output of the I. F. network is then impressed upon a second detector, or audio demodulator. The demodulated energy is utilized in one or more stages of audio amplication, and then finally reproduced. The receiving system could be employed as a portable receiver in an automobile, or in a cabinet in the home of the user. Ihe operating range of the receiver can be in the broadcast range, or the receiver may be of the multi-range variety, and in that case it could also be Operated in the various short wave bands outside the broadcast band.

The automatic volume control system is utilized in the receiver, as is well known at the present time, in order to maintain a substantially constant energy level at the input of the audio demodulator. Because of fading phenomena, or other causes which result in effects analogous to fading, the signal energy level at the audio demodulator tends to fluctuate. The automatic volume control system overcomes this tendency, and for this reason is utilized in modern radio receivers. A

In Fig. lis shown a form lof the present invention, and its purpose is to make the receiver quiet, except when a signal greater than a certain value is tuned in. The receiver is, of the superheterodyne type, as stated before, Aand the automatic volume control system is of any conventional type. For example, any of the automatic volume control arrangements disclosed in the aforesaid parent application can be used for the AVC system. In general, the system employs a rectifier for developing a control voltage from energy of intermediate frequency, for eX- ample, and the control voltage is used to bias the radio frequency amplifiers, and the intermediate frequency amplifiers if desired, in the proper direction to maintain a substantially constant signal amplitude at the circuit |00, which is the signal input circuit ofthe second detector.

The background noise suppressor employed in conjunction with the AVC system is embodied in the second detector network which comprises the tube IOI, and the tube includes a plate, a cathode and five intermediate grids. The tube IOI is of the 6A'7 type, and its signal grid |02 1 is connected to the high alternating voltage side of input circuit |00, while the low alternating voltage side of the input circuit is connected to the grounded side of the bias resistor |03. The plate of the tube is connected to a source of positive plate voltage through a path which includes lead |04 and coil |05, the coil |05 being shunted by a condenser I 06.

The coil |05 is magnetically coupled to a coil |01, the latter vbeing shunted by a condenser |08. One side of coil |01 is grounded, and its other side is connected to grid |09 through a condenser IIO. Grid III is connected to a source of positive voltage through the primary winding of transformer ||2, the secondary winding of this transformer being connected to the audio utilization network of the receiver. A by-pass condenser IIS is connected between the grid |I| and the grounded side of resistor |03, a condenser ||4 shunting the resistor |03. The grid |09 is connected to the high potential side of resistor |03 through a resistor I5, and the signal grid |02 is disposed between a pair of positive screen grids adapted to be connected to a source of positive voltage. The grid bias on the signal grid |02 isl determined by the drop vacross resistor |03. This resistor is made sumciently large so that the bias on Vthe signal grid |02 is more negative than the cut-off value. Weak signals cannot cause any plate current to flow in the tube. When the signal voltage is larger, however, some plate current will flow, and this, in turn, will cause the signal voltage to appear in the tuned circuit I05--I06 disposed in the plate circuit of tube I0|. Because of the magnetic coupling between circuits IUS-|06 and I01-|08, when signal voltage y'appears in the tuned circuit IDE-|00, such signal voltage will be impressed on grid |09.

By virtue of the fact that grid |09 does not have any bias impressed on it, grid current will be caused to flow, and the flow of grid current will bias oi this grid, and thereby reduce the current from the cathode of the tube. This, in turn, willcause the drop in the cathode resistor |03 to become less, and the bias on grid |02 is, therefore, made less and the signal will be amplified more. The action is accumulative, so that the change will continue until the tube is amplifying the signal to its full extent, and audio voltagevwill appear in the transformer II2 connected to grid When the receiver is detuned, the action will take place in the reverse direction, so that the background noise will not be suicient to trigger olf the tube, and no audio output will be produced across transformer |I2. It is important 4to have the windings in the plate intermediate frequency transformer i5-|07 in such a direction as to make the grid |09 of opposite phase to that of the plate.

It will, therefore, be observed that in Fig. 1, the intermediate frequency component of second detector plate current is utilized to control the effective bias on the signal grid of the second detector, and that an intermediate grid of the second detector tube is employed as the audio output detector of the system. In other words, an alternating current is utilized to energize the cold electrode of a noise suppressor control section; in the present case the alternating current is of intermediate frequency.

In Fig. 2, is shown a receiver of the type ernploying AVC, the background noise suppressor being adapted especially for A. C. operated receivers. The background noise suppression function, which is used to supplement the AVC sys- .tem and the latter being conventionally shown,

is performed With only one eXtra tube. The auxiliary tube is of the diode-triode type and is denoted by the reference numeral 200, the latter preferably having a high amplification constant. With ordinary noise suppression circuits the rectiiied voltage available must be high in the receiver in order to furnish voltage to the noise suppressor tube. Also, the heater of the noise suppressor tube must be separate from the other heaters because cf the voltage difference between the cathodes. With the arrangement shown in Fig. 2 no extra voltages are required.

The signal, in this case of intermediate frequency, is impressed on the diode rectifier associated with the tube 200 through the sharply tuned network 20|. The grid of tube 200 is connected to the negative side of the load resistor 202, the latter being shunted by a by-pass condenser 203. The cathode of tube 200 is connected by lead 204 to a point Z in the power supply network. This point is intermediate the resistor 205 land the series arranged filaments 20S. The numeral 201 designates the usual rectier associated with the A. C. voltage line, and it is to be understood that the filaments 206 represent the various laments of the receiving system. The reference character F1 denotes the usual filter network associated with the rectified alternating current.

The numeral 208 designates the bleeder resistor connected across the lter output F1. The positive side of resistor 208 is connected to the plate of tube 2| 0, which in this case is shown as a second detector. It is to be clearly understood that the tube 2|0 can, also, be an audio amplier tube, since the noise suppression action is performed on the tube 2I0. The screen grid of tube 2I0 is connected to the lead 2| I, and a resistor 2|2 isV disposed in the lead to the plate of tube 2|0. The numeral 2I3 designates the path through which the audio component of the second detector output is impressed upon a following audio amplier network.

The plate of tube 200 is connected to the cathode of tube 2|0 through a path which includes resistor R and the resistor 220, each of the resistors being shunted by a by-pass condenser. The point A' designates the junction of resistors 220 and R11, while the point B2 designates the plate side of resistor R11. The signal grid of Idetector tube 2I0 is connected toI point B2 through a path which includes resistors 22| and 222 in series, and the junction of these two resistors is connected to the cathode side of resistor 220 through a condenser 223. The reference numeral 224 designates the tuned coupling network between the I. F. amplifier and the input electrodes of the second detector tube 2|0. The negative side of the bleeder resistor 206 is connected by lead 23D to point A.

5 In the absence of signal energy the noise suppresser tube 200 is conductive, and point B2 will be negative with respect to the cathode of tube 2m. This results in a biasing of the grid of tube Zlil so far negative that tube 2li] becomes inoperative. When signals are impressed on the noise suppressor tube, the tube is rendered less conductive. When the impressed signal is sufciently strong the noise suppressor tube becomes non-conductive, and there is no voltage drop across resistor R11.. The second detector is thus permitted to operate in a normal manner. The noise suppressor tube is a self-rectier, so that no extra voltage is required for the main rectifier tube. The A. C. applied toI the plate of the suppressor tube 209 is the drop yacross the v l heaters 236, as willbe seen if the path of the plate current is followed; starting from the plate; through resistor R11; through heaters 20B; back to the cathode of tube Zll. The alternating current applied to the noise suppressor tube can be supplied by a tap on the high voltage secondary of the usual power transformer. If desired, a buzzer coil of a vibrator-rectier unit, such as used on automobile radio receivers, may be utilZed.

While I have indicated and described several systems for carrying my invention. into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In a superheterodyne radio receiver provided with an automatic volume control circuit,

nitude such that the bias of the signal input grid becomes increasingly positive when signals are impressed upon the rst named grid within the tube.

2. In a radio receiver of the type provided with a demodulator, and means for automatically regulating the gain of the networks preceding the l demodulatorl thereby to maintain the signal amplitude at the demodulator input substantially uniform, said -demodulator comprising a tube provided with an anode and a cathode, a signal coupling network between the demodulator anode and a cold electrode disposed in said demodulator tube for impressing alternating current energy of desired signal frequency upon said cold electrode, a second cold electrode disposed within said demodulator tube and connected tc the signal input circuit of the demodulator, a third cold electrode within the demodulator tube, an audio network coupled to said third electrode, a resistor in the space current path of the demodulator tube, means for connecting sai-d second cold electrode to a point on the resistor which is at a negative direct potential with respect to the cathode potential, and said resistor having a magnitude such that the effective bias of the second cold electrode becomes increasingly positive with respect to the lcathode when signals are impressed upon said rst cold electrode.

WINFIELD R. KOCH. 

